The present invention is a fluid pressure transducer having high accuracy and repeatability. It is particularly useful for pore pressure measurement in landfills and in natural alluvial sediments and similar geologic formations.
A knowledge of ground water pressure is important in the design and maintenance of construction projects such as large highway cuts and fills, earth filled dams, and similar engineering projects. Advanced knowledge of ground water pressures enables the project to be designed so as to withstand the most severe pressures which are expected. Knowledge of pore pressures is also critical in monitoring areas which may have a tendancy to slide. When critical levels are approached, measures can then be taken to remove equipment and personnel until conditions are again safe.
It is common practice to bury pore pressure transducers permanently. This can be done either in bore holes made especially for the purpose, or in construction fills as they are aggraded. For this reason, the transducer should be relatively small and inexpensive and not subject to failure from such means as corrosion, fatigue, or other deterioration of operating parts.
Most pore pressure transducers operate on a null balance principle. Water or other fluid in the area where the transducer is installed acts against one side of a flexible diaphragm. Most typically, air or another gas such as nitrogen is supplied from a pressurized supply source to the other side of the diaphragm. At the point where the pressures on the two sides of the diaphragm are balanced some means is provided to indicate the pressure at the point of installation. Simpler devices, such as the one shown in U.S. Pat. No. 3,318,140 to Shields et al., simply insert a long liquid filled tube to the point of measurement. This has a pressure gauge at the above ground end of the tube which gives a rough indication of the underground pore pressure.
Somewhat more sophisticated instrumentation is disclosed in U.S. Pat. No. 3,365,949 to Robinson and U.S. Pat. No. 3,388,598 to Hall. These are of the diaphragm type mentioned above. The transducers are equipped with two lines which run to the above ground measurement point. One serves as a fluid supply line while the other is connected to a pressure gauge or equivalent measurement device. The diaphragm is held against the ends of the tubes by the water pressure at the measurement location. Gas is introduced against the opposite side of the diaphragm through one of the tubes. When the pore pressure is exceeded the diaphragm moves slightly and lifts off the end of the other tube connected to the gauge. Ultimately, a point of equillibrium is reached and the gauge pressure gives a close approximation of the pore pressure. With devices of this type a number of measurements are frequently made in which supply pressure is cyclically increased and decreased around the null point. This is necessary to overcome errors introduced by large pressure drops along the supply tubing.
Devices of the Hall and Robinson type tend to lose some accuracy because the control or inlet fluid acts only against a small portion of the diaphragm. A somewhat different arrangement is seen in U.S. Pat. Nos. 3,456,509 and 3,574,284 to Thordarson. In Thordarson's transducer units the pore pressure acts against the diaphragm which, in turn, holds a ball check valve open. To take a measurement the system is pressurized. When the internal pressure equals the pore pressure the ball check closes. At that time there is no more communication between the pressure supply line and the gauge line. Even though supply line pressure is increased further the gauge pressure remains constant and is an indication of the pore pressure.
Hernandez et al., U.S. Pat. No. 3,950,997 discloses a similar null balance system in which the diaphragm is replaced by a metallic bellows. Hancock et al, U.S. Pat. No. 4,090,397, show a more sophisticated system using three tubing lines between the transducer and the measuring station. One serves as a supply line, the second is a gauge line, and the third is a vent line to the atmosphere. The pore pressure holds a diaphragm actuated seal against the vent line under normal conditions. As the pressure of the control fluid is increased the diaphragm is ultimately moved, thus opening the vent line. Further control fluid applied to the system escapes through the vent line. If flow rates are held reasonably constant, the pressure indicated by the gauge at the above ground end of the gauge line will be an indication of pore pressure.
In order to obtain accurate readings two criteria must be met. It is essential that there be no flow of the pressurizing fluid in the gauge line at the time a reading is taken. The tubing from the transducer to the ground station is normally of very small internal diameter. As noted before, any fluid flow through this tubing is subject to very large pressure drops with resulting inaccuracies in the readings. A second potential source of error is far more subtle and is one which represents a problem only in certain soil environments, such as tight clay formations. Virtually all of the prior art devices require a significant diaphragm movement at or near the null point. In a tight clay formation water movement is very slow and highly restricted. If diaphragm movement at the null point is toward the ambient water there is no place for this water to go and diaphragm displacement will be strongly resisted. If, on the other hand, diaphragm movement at the null point is away from the ambient water, cavitation will result beneath the diaphragm. Either condition causes false readings of pore pressure. To achieve the highest accuracy it is thus essential that diaphragm displacement be held to the minimum possible volume. While this situation may not be a problem in more open formations, the environment of use is a condition over which the manufacturer has no control and he must assume that his instrument will be used in the most unfavorable environments.